[flang] Fix Darwin build after 4762c65

Select POSIX 2008 standard to avoid including Darwin extensions.
Otherwise, Darwin's math.h header defines HUGE, which conflicts
with Flang's HUGE function.

This started happening after 4762c65 (llvm#82443), that added the
"utility" include, which seems to include "math.h".

Author: luporl

flang/CMakeLists.txt

@@ -413,6 +413,14 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
 
 endif()
 
+# Clang on Darwin enables non-POSIX extensions by default, which allows the
+# macro HUGE to leak out of <math.h> even when it is never directly included,
+# conflicting with Flang's HUGE symbols.
+# Set _POSIX_C_SOURCE to avoid including these extensions.
+if (APPLE)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
+endif()
+
 list(REMOVE_DUPLICATES CMAKE_CXX_FLAGS)
 
 # Determine HOST_LINK_VERSION on Darwin.

flang/include/flang/Evaluate/integer.h

@@ -27,10 +27,6 @@
 #include <string>
 #include <type_traits>
 
-// Some environments, viz. clang on Darwin, allow the macro HUGE
-// to leak out of <math.h> even when it is never directly included.
-#undef HUGE
-
 namespace Fortran::evaluate::value {
 
 // Implements an integer as an assembly of smaller host integer parts

flang/include/flang/Evaluate/real.h

@@ -18,10 +18,6 @@
 #include <limits>
 #include <string>
 
-// Some environments, viz. clang on Darwin, allow the macro HUGE
-// to leak out of <math.h> even when it is never directly included.
-#undef HUGE
-
 namespace llvm {
 class raw_ostream;
 }

flang/lib/Evaluate/fold-implementation.h

@@ -39,10 +39,6 @@
 #include <type_traits>
 #include <variant>
 
-// Some environments, viz. clang on Darwin, allow the macro HUGE
-// to leak out of <math.h> even when it is never directly included.
-#undef HUGE
-
 namespace Fortran::evaluate {
 
 // Utilities

flang/lib/Evaluate/intrinsics-library.cpp

@@ -299,8 +299,8 @@ struct HostRuntimeLibrary<std::complex<HostT>, LibraryVersion::Libm> {
 /// Define libm extensions
 /// Bessel functions are defined in POSIX.1-2001.
 
-// Remove float bessel functions for AIX as they are not supported
-#ifndef _AIX
+// Remove float bessel functions for AIX and Darwin as they are not supported
+#if !defined(_AIX) && !defined(__APPLE__)
 template <> struct HostRuntimeLibrary<float, LibraryVersion::LibmExtensions> {
   using F = FuncPointer<float, float>;
   using FN = FuncPointer<float, int, float>;